For molding, with an injection molding machine, such products as lenses and discs having a relatively large thickness and required to have precise configurations and small internal strains, an injection molding machine has been used capable of injecting a large quantity of molten material in each shot such that the quantity of the molten material injected into a mold cavity of a metal mold would not be deficient. For this reason, a method and apparatus have been used wherein a movable metal mold is positioned in a stationary metal mold with a small gap or compression clearance left therebetween, molten resin is filled in the mold cavity defined between the movable and stationary metal molds, and then the movable metal mold is advanced to close the gap or reduce to zero the compression clearance. However, this compression movement is started immediately after filling the molten resin into the mold cavity, or a predetermined time after completion of the filling of the molten resin. For this reason, the compression starting time substantially lags the filling starting time with the result that the molten resin injected into the mold cavity solidifies starting from the contact surface between the resin and the metal mold. Accordingly, when the compression is continued under this condition, it is inevitable to create an internal stress in the molded product.
In the prior art method wherein the compression clearance is reduced after filling the molten resin, a large mold clamping force is necessary thus increasing the size of the injection molding machine. Where a molding machine requires a small mold clamping force but it is necessary to increase the quantity of the injected resin per shot, the screw is rotated while being advanced toward the metal mold.
With this method, however, the molten resin is conveyed into the mold cavity only by the transfer force caused by the rotation of the screw so that where the cavity has an intricated configuration, the resin cannot reach deep portions of the cavity. Moreover, as the filling speed of the resin is relatively low, there is the same defect that the resin begins to solidify starting from the contact surface between the filled resin and the metal mold.